One early form of "cheating" was known as "banner farming", where websites would open separate sub-windows which were invisible to users and hosted ads that were never seen.

"When fraudsters manipulate advertising systems, their efforts often may appear highly effective but, in fact, invite advertisers to invest in placements that are ineffective," Edelman said.

Detecting this behaviour was complicated by the low average clickthrough rates recorded by display ads as a whole – typically coming in at around 0.1%.

Pay-per-click techniques appear to be a step forward in this area, but some third-party services hired by advertising networks to syndicate ads then use tools like botnets and adware to falsify clicks.

Relying on conversion rates – namely, the number of actual purchases made – might be one way to reward effective sites. It threatens, however, to undermine platforms where influence is real, but not immediate.

"Cheaters" could also "game" such approaches by identifying buyers predisposed to making purchases from a certain site or vendor, and then claiming to have referred them.

In terms of measurement, a current weakness involves the difficulty of discounting those users clicking on ads solely for convenience, as they would have chosen a particular site anyway in organic search listings.

"The better known an advertiser, the greater the chance that buyers would buy at its site even without advertising," Edelman suggested.

And the incentives for discovery, he continued, are "mixed" for some key players within the digital ecosystem.

"Advertising brokers and networks have mixed incentives," Edelman wrote. "They sometimes genuinely seek to improve advertisers' efforts, but often their interests are at odds with what's best for advertisers."

Furthermore, he insisted, "Standard methods of accountability and dispute resolution have proven ineffective; advertisers have struggled to hold perpetrators and intermediaries accountable for apparent breaches."